Milling screw-threads



G. RICHARDS, L. N. BURT AND I. E. FREEBORN.

MILLING SCREW THREADS. APPLICATION man APR. 8 P920.

1,386,279. Patented 2, 1921.

3 SHEETS-SHEET l- "-4 d 71 I f Q4 '11; g win l I 'I r d I Fig. 4

I: i? l i; g i fi h 5 ii is J E G. RICHARDS, L. N. BURT AND I. E. FREEBORN. MILLING SCREW THREADS.

APPLICATION FILED APR. 8. I920.

1,386,279., Patented 2, 1921.

3 snzzfs-sussr 2. .5

- I IllllllllllIlIllI llllllllllllllll G. RICHARDS, L. N. BURT AND J. E. FREEBORN.

MILLING SCREW THREADS.

APPLICATION FILED APR. 8. 1920.

1 86,279 Patented Aug. 2, 1921.

3 SHEETS-SHEET 3- Fig. 6.

INYENTOPS k- M l 2. 39m

UNITED STATES PATENT OFFl'CE.

GEORGE RICHARDS AND LESLIE NEWMAN BURT, OF WESTMINSTER, LONDON, AND JAMES EDWARD FREEBORN, 0F \VALTI-IAMSTOW, ENGLAND, ASSIGNORS TO-THE RICHARDS THREAD MILLING MACHINE COMPANY, (1918,) LIMITED, OF WES MINSTER, LONDON, ENGLAND.

MILLING SCREW-THREADS.

Application filed April 8,

To all whom it may concern:

Beit known that we, GEORGE RICHARDS, a citizen of the United States, and Lnsrm NEWMAN BURT, a subject of the King of Great Britain and Ireland, both residing at The Outer Temple, 222 Strand, in the city of Westminster, London, England, and JAMES EDW RD FREEBORN, a subject of the King of Great Britain and Ireland, residing at 2 Elmsdale road, \Valthamsto-w, in

'the county of Essex, England, have invented new and useful Improvements in MilIing ScreW-Th'reads, of which the following is a'specification.

' This invention relates to an. improved method of cutting screw-threads with the aid of what are known as hobs; that is .to say, rotary cutters wherein the cutting teeth follow spiral lines having a pitch corresponding with that of the thread to be formed It has hitherto been the practice to impart to the hob and to the work-piece the same rotative speed, but neither partakes of any axial movement. Under such conditions, it is necessary to make special provision for elimination of facets, which result from the cutting teeth coming successively into operation upon the work at the same points in successive revolutions. There is, moreover, the objection that the portion of the work-piece threaded is restricted in length to that of the hob. A further objection is that, when using a multiple-thread hob, difiiculties occur in shaping the toothed thread on the hob in such a manner as to cut, in the work, a thread of the correct sha e.

ith a view to avoiding these objections,

we mount the hob-spindle in a frame capable ,of being canted; such frame being mounted on a slide, so as to admit of the hob being advanced laterally toward the workpiece with a view to cutting thethread in the latter to' the required depth. The said slide. is. in turn mounted ona saddle, adapted to be traversed upon the bed of the machine, andis provided with a screw of predetermined pitch for effecting such movement. A When the hob-spindle is tilted in a vertical plane, and when extreme accuracy is required-in the form of the thread cut,

Specification of Letters Patent.

Patented Aug. 2, 1921.

1920. Serial No. 372,315.

the surface of the hob is made slightly curved in a longitudinal direction, or, for internal work, slightly b: rrel-shaped.

The work-spindle, which is mounted in the headstock of the machine, is in, gear with the hobspin dle; these two spindles revolvlng, contrary to the usual practice, at slightly difi'erent speeds. The work-spindle is also in gear with the screw which operates the traverse slide, the pitch of the-said screw, or the gearing whereby it is driven, being proportioned to the difl'erence between the rotative speed of the work-spindle and that of the hob-spindle. 7

Under such conditions, by reason of the axial advance of the hob in relation to the work, the rotative speed of the former, or of the latter, being varied to suit, we are able to screw-thread portions of the work the length of which considerably exceeds that of the hob. Moreover, as work-threads cut by the leading hob-threads are subsequently operated on by the following hob-threads, any facets formed by the leading hol threads become eliminated.

The provision of substantial and rigid support for the cutter-spindle is of importance, and is facilitated by the employment of a taper hob. 1

One mode of constructing apparatus adapted for use in carrying our invention into effect is illustrated in the accompanying drawings, whereof Figure 1 is a vertical section of part of the gear employed to drive the cutter spindle, Fig. 2 being a partly diagrammatic end elevation illustrating gears arranged between the latter and a traversing shaft hereinafter referred to. Fig. 3 'is a part plan illustrating certain further details of'the traversing mechanism, while Figs. 4; and 5 are part views in plan illustrating the application of our invention where a taper hob is employed. Figs. 6 and .7 are respectively a sectional plan and a front, elevation showing the general arrangement of the apparatus, while Fig. 8 is a diagrammatic .view illustrating certain further gearing hereinafter described.

By transmitting to the cutter-spindle by wayor a suitably arranged differential gear, the rotative movement which is imparted to the carriage-traversing mechanism, the hob may be speeded up, or retarded, according to the direction of travel, in such a manner that the relation between the hob-threads and the threads on the work remains undisturbed when the hob is moved longitudinally; thus enabling the workpiece to be screw-threaded over a length greatly exceeding that of the hob.

As shown in Fig. 1, the drive is transmitted from the back-shaft a to a worm- Wheel a mounted on a bearing attached to a saddle b. In a suitable bracket 6 mounted upon the worm-wheel a and able to revolve with the latter, is arranged a short shaft 0 whereon' are freely mounted two similar bevel-wheels c 0 which mesh with a bevel-wheelr d, mounted on a shaft'd whereof the "outer extremity carriesa spurwheel d The bevel-wheels c 0 also mesh with a further bevel-wheel e, mounted upon a cross shaft f, from which the cutter spindle is driven. From the back-shaft a drivingtlie worm-Wheel a the drive is transmitted to the cross-shaft f, by way of the bevel-wheels c c meshing with the bevel-wheel cl, and

thence to the bevel-wheel e, which is fast upon the cross-shaft f; the bevel wheelv d remaining locked when, as in cutting a short length ofscrew-thread, no traverse of the hob is eifected.

The traverse of the saddle b, and hence that of the hub, is effected by transmitting the power supplied by the back-shaft a through suitable gears'g, g 9 as shown in Fig. 2, to drive the worm-shaft h, at a suitable speed; provision being made for throwing these gears into-and out ofengagement. The worm-shaft h, carries a worm h which meshes with a worm-wheel 71. the boss of which is internally screwthreaded to form a nut adapted totraverse the lead-screw j, which is held against rotation. The outer extremity of the wormshaft-h, carries a spur-wheel 7L3, which,as shown in Fig. 3, meshes with the spurwheel d through one or more suitable idlers, such as (Z provision being, moreover, made for varying the said spur-wheels and idlers in order to establish different speed ratios adapted to produce threads of the particular form required.

Short lengths of thread, less than or equal to thehob in length, may thus be cut without any endwise motion of the saddle b,

' when the feed drive is, by any suitable means, thrown out of engagement; but should a creepfimotion be required in these short lengths of thread, a'hand -lever may be attached to. the front end of the wormshaft. hand the required creep be produced by hand. V

As an alternative, the lead-screw may be driven in conjunction with similar gear to produce the same effect; or the lead-screw may be driven direct producing a different speed ratio from that of the hob, as in Fig. 5. V r

In order to permit of the cutter-spindle being rigidly supported, a taper hobmay be employed, so as to permit of the carriage assuming an angular position in relation to the work; thus providing the necessary clearance for a substantial and steady carriage, as shown in Figs. 4 and 5.

To enable the hob to assume the correct position for the teeth to cut in the path of the helix angle of the thread on the work, the saddle b is, as shown in Fig. 6, mounted on a bed and carries a cross-slide Z which may be moved laterally, to obtain various depths of thread, by means of a screwspindle m.

The cross-slide b" carries a. pivotally mounted carnage 1%, arranged vertically in relation to the bed and .adaptedto rotate about the center 70 ,as shown diagramma th cally in Fig. 8, inorder to adjust the axis of the hob to any angle required. i j

The carriage a which supports the cutter spindle 79 may, by meansof the screw 0,

and hand-wheel 0 be adjusted vertically on slides carried by the pivotally mountedcan rlage n, asillustrated in Fig. 7. -,The drive for the cutter-spindle p is. imparted,

shown in Fig. 6, from the cross-shaft f through suitable spur-wheels p 39?, p .10 whereof the last indicated is free to revolve about the center 70 ,.whereon the spur-wheel p is mounted.

The spur-wheels 39 79 10 are mounted in the cross-slide Z; thespur-wheelerbeing free to slide upon a spline along the cross-shaft f. V I I.

Thespur-wheel p drives the worm-shaft rshown in Fig. .6, and thence, by way of the worm r and the worm-wheel 8 the vertical shaft upon which the worm-wheel t is free to slide vertically upona spline, as

shown in Fig. 7 thev cutter-spindle p bemg ClllVGII by means of a worm-wheel w:

mounted at its extremityvand meshing with the worm-wheel t. V

Under such conditions, the hob-teeth may be'adjusted to out along any angle required to conform with the helix of any particular thread. Furthermore,*on reaching the end of the thread when using a parallel hob,

the carriage a may be fed 'downward to finish those threads, which, by reason of' the end of the hob being canted above the work-center, have been left unfinished.

We claim 1. The herein described method ef mill ing screw-threads with the aid of .ahob, consisting in imparting to the hob a gradual axial motion, the relative rotative speeds ofthe hob and of the work-piece although nominally equal being. suitably :adjusted to compensate for the axial motion of the hob.

2. The herein described method of milling screw-threads with the aid of a hob, consisting in imparting to the rotating hob a gradual axial motion and imparting to the work-piece an additional relative rotative speed of one revolution for each advance of the hob for a distance equal to the pitch of the thread cut.

3. The herein described method of milling screw-threads over a length substantially greater than the axial length of the hob employed consisting in mounting a taper hob on a spindle inclined to the axis of the work piece and borne in a traversing carriage to which a gradual longitudinal motion is imparted; the relative rotative speeds of the hob and of the Work-piece, although nominally equal, being suitably adjusted to com- Ipensate for the longitudinal advance of the GEORGE RICHARDS. LESLIE NEWMAN BURT. JAMES EDWARD FREEBORN. 

